Added more Doxygen comments everywhere.
Lots of structural and memory-related fixes.

1. Fixed the logic of adding/removing entities.

2. Fixed the entire destroy operation chain that was broken.

3. Fixed a memory corruption in the 2D dynamics engine (you must first free the shape, then the body; can't do the opposite).

4. Restored the <distribute> tag functionality.

After a few tests with valgrind, ARGoS seems memory-corruption-free.